NMR spectroscopy, the process of analyzing a small sample in a uniform magnetic field and obtaining radiofrequency data resulting from precisely pulsed radiofrequency excitation, was invented by Block and Purcell. In the last 15 years, NMR analysis by spectroscopy has shifted from physical chemistry to biological chemistry and biomedical applications; i.e. biopsy samples of normal and diseased tissues.
Separately Lauterbur and Damadian invented the utilization of NMR principles to produce an image. The resulting devices, NMR imaging systems, produce two-dimensional and three-dimensional data where the grey scale represent, the three parameters nuclide density, and T.sub.1 T.sub.2 relaxation parameters spatially in an anatomical image.
The measurement of magnetic field distributions is a requirement of many experimental applications. It is particularly important in the area of nuclear magnetic resonance (NMR) imaging, where an increasing number of clinical applications are being studied and NMR imaging systems are being developed commercially. In these systems the image quality is greatly dependent upon the strength and homogeneity of the magnetic fields used, influencing the sensitivity of the data collection and the accuracy of the spatial measurement. A necessary requirement for the successful installation of each new imaging system is the rapid setting or shimming of the magnet. The observation of the field distributions during the shimming procedure is frequently done by a point by point measurement of the field using a NMR probe. This method is, however, a relatively inefficient procedure, and there exists an experimental method which is based on the principles of Fourier imaging (A. Kumar, D. Welti and R. R. Ernst, 1975 "NMR Fourier zeugmatography" J. Magn. Reson. 18, 69-83 and A. A. Maudsley, A. Oppelt and A. Ganssen, 1979 "Rapid measurement of magnetic field distributions using nuclear magnetic resonance" Siemens Forsch u. Entwickl. Ber. 8, 326-31) that provides a faster, more accurate, and more convenient means of measuring the magnetic field homogeneity in two or three dimensions.
In-vitro sample NMR magnets for NMR must have 0.1 ppm homogeneity or better in the sample area to provide a useful resolved spectrum. A whole body in-vivo NMR magnet at today's state of the manufacturing art over a head or body organ size volume is at best 5.0 to 10.0 ppm. Since peaks are separated by 2 to 5 ppm at levels of medical significance, 5 to 10 ppm is insufficient for meaningful NMR spectroscopy.
In the use of NMR spectroscopy, a clinician must acquire spectra from a desired location within a patient's organ without intervention (biopsy); i.e. compare normal tissue with suspected disease tissue. Spectra can be obtained from the total image volume or area or line of NMR image data but this has not been sufficient to localize the information. The use of a small surface coil placed on the surface of the body has been suggested to obtain spectra. However, this does not provide sufficient localization, either in acquired spectral data due to interlying tissue between the coil and the desired point of sensitivity, or in any guide to anatomical location as to where to place the coil to obtain spectra from a desired location.
For effective diagnosis, it is necessary to provide visual communication of spectral information to the image-oriented diagnostician. A display of a spectrum per pixel or voxel, however, in accordance with prior practice, would be both confusing to an image-oriented diagnostician as well as presenting too much data to be meaningfully absorb for diagnostic purposes.